Numerically minor subsets of lymphocytes have been found to have potent immunomodulatory functions. Indeed, the identification of T cell subsets such as regulatory T cells, T follicular helper cells, natural killer T cells and IL-17 producing T cell have, cumulatively, represented one of the most important advances in understanding the adaptive immune response. BTB-zinc finger (BTB-ZF) transcription factors are defined by an N-terminal protein-protein interaction domain (BTB/POZ) domain, coupled with a C-terminal zinc finger domain. Several members of this BTB-ZF family have emerged as fundamental, non-redundant factors that control the development or function of specific cell types of the immune system. For example, BTB-ZF genes have been shown to control T cell versus B cell commitment (LRF), CD4 versus CD8 lineage commitment (ThPOK), the commitment of thymocytes to innate T cell lineages (PLZF), the development of T follicular helper T cells (Bcl6) as well as differentiation of B cells into germinal center B cells (Bcl6). Additional members of ths gene family clearly influence the immune response, but in less well defined ways. In this R21 application we propose to explore BTB-ZF gene family expression to define novel leukocyte subset effector populations. We will develop a set of BAC-based transgenes that express both GFP and Cre under the control of the regulatory elements for BTB-ZF genes. Preliminary studies suggest that each of the selected BTB-ZF genes is expressed in the immune system, is regulated during development and, potentially, controls the function of a novel subset of cells. We are well positioned to carry out these studies and have already proven that the BAC reporter system is a highly effective approach. Indeed, we would strongly argue that this single cell expression model is essential for identifying leukocyte subsets and for the study of this important new family of transcription factors. The dramatic and nonredundant functions of the BTB-ZF genes that have thus far been identified clearly support the significance of the reagents we propose to develop and evaluate in the context of this R21 application. We believe that this effort will significantly advance the understanding of the transcriptional control of immune responses. Finally, since this family of transcription factors is highly conserved in humans, our studies have the potential to lead to new diagnostic tests and/or therapies.